cql3/maps: do not assume that the term type is constants::value
cql3/maps: do not assume that the term type is constants::value

Fixes #516.

Signed-off-by: Paweł Dziepak <e29bb3265626c60caf76d587cabdfcd8b379ef80@scylladb.com>

avikivity/scylla,kjniemi/scylla,duarten/scylla,kjniemi/scylla,raphaelsc/scylla,kjniemi/scylla,raphaelsc/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,raphaelsc/scylla